Method for compressing a gas, computing unit and multi-stage piston compressor

ABSTRACT

A method for compressing a gas by means of a multi-stage piston compressor is disclosed, wherein, if an inlet pressure of a first compression stage exceeds a threshold value, the gas is at least partially, in particular completely, branched off before the first compression stage and fed to a second compression stage, which directly follows the first compression stage. A computing unit for performing the method to such a multi-stage piston compressor are further disclosed.

The invention pertains to a method for compressing a gas by means of a multi-stage piston compressor, a computing unit for carrying out said method, as well as such a multi-stage piston compressor.

PRIOR ART

Devices for compressing gases are generally known. For example, reciprocating piston compressors, rotary compressors or ionic compressors can be used for this purpose. Higher compression ratios can be achieved, for example, by using a multi-stage configuration of compression devices. In this case, individual stages usually separated from one another by valves or gate valve gears.

With respect to their performance and the power, such compression devices or compressors are typically designed for defined parameters such as pressures, piston diameters and therefore also gas forces. It is therefore frequently difficult to realize a change of these parameters on existing systems.

With respect to their power, compressors with drives in the form of linear motors, particularly electric linear motors, may be designed such that the compressive force of the individual compression stages is higher than the attainable maximum power of the linear motor. An operational superposition of the oscillating mass force and the compressive forces results in a maximum force that has to be lower than the maximum attainable power of the linear motor. In this way, high pressures can also be generated by small driving mechanisms without having to use small piston diameters and thereby forfeiting capacity.

In compressors that are operated near the limit of the maximum attainable power, however, a shutdown typically takes place when the inlet pressure of the gas to be compressed increases because the attainable power of the compressor no longer suffices for compressing the gas as intended.

It would therefore be desirable to provide an option for preventing a shutdown in such compressors during an increase of the inlet pressure.

This objective is attained by means of a method for compressing a gas, a computing unit for carrying out said method and a multi-stage piston compressor with the characteristics of the independent claims.

Advantages of the Invention

The inventive method serves for compressing a gas by means of a multi-stage piston compressor. If an inlet pressure of the first compression stage exceeds a threshold value, the gas is at least partially, in particular completely, diverted upstream of the first compression stage and fed to a second compression stage, which directly follows the first compression stage.

This makes it possible to prevent gas with a pressure, at which the compression in the first compression stage would no longer be possible, e.g., due to an unattainable power of the drive used, from being fed to the first compression stage in the first place or at least from being completely fed to the first compression stage. The first compression stage therefore does not have to be shut down. The diverted gas is instead directly fed to the following compression stage. Since this compression, stage is designed for higher inlet pressures, the gas can be compressed therein. It is therefore also possible to use a compressor for higher inlet pressures than originally intended. This opens up additional applications for a compressor, which previously would have required a larger or more powerful compressor.

Furthermore, the stroke of a piston of the piston compressor, which is assigned to the first compression stage, is advantageously reduced. This makes it possible to intentionally create a clearance volume in the first compression stage in order to increase the gas quantity in the corresponding cylinder. An excessively low cylinder pressure in the first compression stage can thereby also be prevented (the compression device ejects gas through an outlet valve while the inlet valve is closed, but no additional gas inflow can take place and the remaining gas is expanded such that a pressure drop occurs in the cylinder), wherein such an excessively low cylinder pressure could lead, e.g., to a shutdown because air would otherwise be drawn in from outside and mix with the gas to be compressed. In addition, the power consumption of the compressor can thereby also be reduced.

The stroke is advantageously reduced in dependence on a residual pressure after a reexpansion in the first compression stage and/or an inlet pressure of the second compression stage. For example, the higher the residual pressure after a reexpansion in the first compression stage and/or the higher the inlet pressure of the second compression stage (referred to the stroke reduction), the smaller the amount, by which the stroke is reduced. In this case, the clearance volume of the respective cylinder of the first compression stage may particularly also be taken into consideration, in this way, the operation of the compressor can be optimally adapted to the respective circumstances. With respect to more details on a potential correlation between a reduction of the stroke and the residual pressure after a reexpansion in the first compression stage and the inlet pressure of the second compression stage, we refer to the description of the figures.

It is advantageous if the reduction of the stroke is determined based on stored the values for the residual pressure and/or the inlet pressure of the second compression stage. In this way, the computing effort can be significantly reduced, particularly in real time. To this end, the values for the stroke reduction may be stored, e.g. in a control unit for the compressor, in increments corresponding to 0.5 bar of the respective pressures.

The gas is advantageously diverted upstream of the first compression stage and fed to the second compression stage in that a first valve in a first flow path leading to the first compression stage is at least partially closed and a second valve in a second flow path leading from the first flow path to the second compression stage is at least partially opened. Such a valve configuration with a second flow path in the sense of a bypass line allows a particularly simple implementation of the method.

An inlet valve of the first compression stage is advantageously used as first valve. For example, the inlet valve is simply shut in order to be closed. In this way, no additional valve other than the second valve is required in the second flow path.

It is advantageous to use at least one electric linear motor for moving pistons in the piston compressor. This allows a particularly simple adaptation of the piston stroke.

An inventive computing unit is designed for carrying out the inventive method. Such a computing unit may be realized, e.g., in the form of a stored program control (SPC). In this case, the computing unit is particularly designed for acquiring and processing the required parameters and for correspondingly activating the required components.

In addition, a knock detection method and a corresponding device are advantageously proposed.

An inventive multi-stage reciprocating piston compressor comprises a gas inlet, a first compression stage and a second compression stage. In this case, a first valve is arranged in a first flow path leading to a gas inlet of the first compression stage and a second flow path branches off the first flow path upstream of the first valve and leads to a gas inlet of the second compression stage. A second valve is arranged in the second flow path.

The first valve is preferably formed by the inlet valve of the first compression stage.

The multi-stage reciprocating piston compressor advantageously comprises at least one electric linear motor for moving pistons of the reciprocating piston compressor.

It is advantageous if the multi-stage reciprocating piston compressor comprises an inventive computing unit.

With respect to advantages and embodiments of the inventive multi-stage reciprocating piston compressor, we refer to the above-described implementations of the inventive method in order to avoid unnecessary repetitions.

The piston compressor is used for compressing gases. The piston compressor is particularly used for compressing carbon dioxide, hydrogen, methane, natural gas, helium or nitrogen.

The reciprocating compressor for compressing gas is preferably operated at temperatures between −253 and 150° C. The piston compressor preferably can compress the gas to pressures between 0.1 bar and 1000 bar.

The temperatures and pressures are dependent on the gas to be compressed.

Preferred operating parameters for compressing different gases with the piston compressor are described below with reference to the following examples. Under certain circumstances, the gases may also consist of wet and/or contaminated gases or of gas mixtures.

Carbon Dioxide:

The inlet temperature of the carbon dioxide (temperature prior to the compression) preferably lies between −60° C. and 120° C., particularly between 1 and 80° C. The outlet temperature of the carbon dioxide (temperature after the compression) preferably lies between 40 and 150° C., particularly between 60 and 100° C. The inlet pressure of the carbon dioxide (pressure prior to the compression) preferably lies between 0.1 bar and 10 bar, particularly between 0.2 and 4 bar. The outlet pressure of the carbon dioxide (pressure after the compression) preferably lies between 5 and 100 bar, particularly between 20 and 60 bar. The volumetric flow rate preferably lies between 0.5 Nm³/h and 50 Nm³/h, particularly between 1 Nm³/h and 8 Nm³/h.

Hydrogen:

The inlet temperature of the hydrogen (temperature prior to the compression) preferably lies between −253° C. and 80° C., particularly between −253° C. and −80° C. when the compressor is used in the form of a cryogenic compressor or particularly between −20° C. and 80° C. when the compressor is used in the form of an ionic compressor. The outlet temperature of the hydrogen. (temperature after the compression) preferably lies between −250 and 150° C., particularly between −60 and 80° C. The inlet pressure of the hydrogen (pressure prior to the compression) preferably lies between 0.8 bar and 40 bar, particularly between 2.5 and 30 bar. The outlet pressure of the hydrogen (pressure after the compression) preferably lies between 10 and 1000 bar, particularly between 500 and 1000 bar. The volumetric flow rate preferably lies between 0.5 Nm³/h and 500 Nm³/h, particularly between 50 Nm³/h and 350 Nm³/h.

Methane or Natural Gas:

The inlet temperature of the methane or natural gas (temperature prior to the compression) preferably lies between −182° C. and 80° C., particularly between −182° C. and −40° C. when the compressor is used in the form of a cryogenic compressor or particularly between −20° C. and 80° C. when the compressor is used in the form of an ionic compressor. The outlet temperature of the methane or natural gas (temperature after the compression) preferably lies between −180 and 150° C., particularly between −60 and 80° C. The inlet pressure of the methane or natural gas (pressure prior to the compression) preferably lies between 0.8 bar and 30 bar, particularly between 1.5 and 20 bar. The outlet pressure of the methane or natural gas (pressure after the compression) preferably lies between 10 and 650 bar, particularly between 300 and 600 bar. The volumetric flow rate preferably lies between 0.5 Nm³/h and 1000 Nm³/h, particularly between 5 Nm³/h and 350 Nm³/h.

Helium:

The inlet temperature of the helium (temperature prior to the compression) preferably lies between −269° C. and 80° C., particularly between −269° C. and −80° C. when the compressor is used in the form of a cryogenic compressor or particularly between −20° C. and 80° C. when the compressor is used in the form of an ionic compressor. The outlet temperature of the helium (temperature after the compression) preferably lies between −269 and 150° C., particularly between −60 and 80° C. The inlet pressure of the helium (pressure prior to the compression) preferably lies between 0.8 bar and 40 bar, particularly between 2.5 and 20 bar. The outlet pressure of the helium (pressure after the compression) preferably lies between 10 and 1000 bar, particularly between 200 and 600 bar. The volumetric flow rate preferably lies between 0.5 Nm³/h and 600 Nm³/h, particularly between 50 Nm³/h and 400 Nm³/h.

Nitrogen:

The inlet temperature of the nitrogen (temperature prior to the compression) preferably lies between −196° C. and 80° C., particularly between −196° C. and −40° C. when the compressor is used in the form of a cryogenic compressor or particularly between −20° C. and 80° C. when the compressor is used in the form of an ionic compressor. The outlet temperature of the nitrogen. (temperature after the compression) preferably lies between −195 and 150° C., particularly between −60 and 80° C. The inlet pressure of the nitrogen (pressure prior to the compression) preferably lies between 0.8 bar and 30 bar, particularly between 1.5 and 17 bar. The outlet pressure of the nitrogen (pressure after the compression) preferably lies between 10 and 650 bar, particularly between 200 and 400 bar. The volumetric flow rate preferably lies between 0.5 Nm³/h and 500 Nm³/h, particularly between 5 Nmj/h and 350 Nm³/h.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an inventive multi-stage reciprocating compressor according to a preferred embodiment in the form of a flowchart.

FIG. 2 schematically shows stroke profiles of an inventive multi-stage reciprocating compressor according to a preferred embodiment in the form of a diagram.

EMBODIMENTS OF THE INVENTION

FIG. 1 schematically shows an inventive multi-stage piston compressor 100 according to a preferred embodiment in the form of a flowchart. In this case, the piston compressor 100 comprises a first compression stage 110 and a second compression stage 120. Both compression stages are respectively realized in the form of pistons that move in a cylinder. These pistons are driven by an electric linear motor 130. Additional compression stages may naturally be provided.

The first compression stage comprises an inlet valve ill and an outlet valve 112, which may be realized in the form of pressure-controlled check valves. The second compression stage 120 likewise comprises an inlet valve 121 and an outlet valve 122, which may also be realized in the form of pressure-controlled check valves.

In this case, the regular gas flow takes place along a first flow path 161 (illustrated on the left in FIG. 1) leading to the first compression stage 110 and then from the first compression stage 110 to the second compression stage 120. Subsequently, the gas can be fed to a desired application.

Pressure sensors 141, 142 and 143 are furthermore provided. An inlet pressure of the first compression stage can be measured with the pressure sensor 141, an outlet pressure of the first compression stage 110 or an inlet pressure of the second compression stage 120 can be respectively measured with the pressure sensor 142 and an outlet pressure of the second compression stage 120 can be measured with the pressure sensor 143. In this case, the pressure sensors 141, 142 and 143 are connected to a computing unit 170, which is realized in the form of a stored program control (SPC). The SPC 170 can therefore respectively acquire or read out the corresponding pressures.

A first valve 151 is furthermore provided in the first flow path 161. This first valve 151 can presently be activated, i.e. opened and closed, by means of the SPC 170. In this case, the first valve 151 is open during the normal operation.

A second flow path 162 in the sense of a bypass line is furthermore provided, wherein this second flow path branches off the first flow path 161, namely upstream of the first valve 151, and leads to the second compression stage 120. A second valve 152, which can likewise be activated, i.e. opened and closed, by the SPC 170, is provided in the second flow path 162. This second valve 152 is closed during the normal operation.

For example, the first valve 151 is completely closed if the SPC 170 respectively acquires or reads out an inlet pressure of the first compression stage 110, which lies above a threshold value, from the pressure sensor 141 during the operation of the piston compressor 100. The second valve 152 simultaneously is completely opened. The gas now flows directly to the inlet of the second compression stage 120 instead of to the first compression stage 110.

This threshold value can preferably be chosen in such a way that the output or the attainable power of the electric linear motor 130 for the first compression stage 110 just barely suffices for carrying out the required compression at inlet pressures below this threshold value. Inlet pressures, at which the required compression can no longer be carried out, are thereby prevented in the first compression stage 110.

The electric linear motor 130 is furthermore activated by the SPC 170 in such a way that the stroke of the piston assigned to the first compression stage 110 is reduced.

FIG. 2 schematically shows stroke profiles of an inventive multi-stage reciprocating piston compressor according to a preferred embodiment in the form of a diagram. In this diagram, the stroke h is plotted as a function of the time t.

In this diagram, h₁ denotes the stroke profile of the piston assigned to the first compression stage and h₂ denotes the stroke profile of the piston assigned to the second compression stage.

The stroke h₁ of the piston of the first compression stage is now reduced by an amount Δh such that the piston of the first compression stage now has a stroke h′₁. The stroke of the piston of the second compression stage remains unchanded. As initially mentioned, a negative pressure in the second compression stage is thereby prevented.

The amount Δh, by which the stroke is reduced, can be calculated based on the following formula:

${\Delta \; {h\left( p_{1} \right)}} = {\frac{\left\lbrack {{\left( \frac{p_{1}}{p_{2}} \right)^{\frac{1}{\kappa}} \cdot V_{stat}} - V_{stat}} \right\rbrack}{\frac{d^{2}}{4}\pi}.}$

In this formula, p1 denotes the residual pressure after the reexpansion in the first compression stage. The pressure p1 may be a freely definable pressure that should preclude an absolute pressure from falling short of 1 bar. In this context, p1>>1 bar absolute should preferably apply. For example, the pressure after the reexpansion may be determined computationally or ascertained indirectly if the pressure p1 drops below the pressure measured by the pressure sensor 141 in the reexpansion period. In this case, gas from the volume between the valves 151, 111 and the pressure sensor 141 would flow in such that the pressure measured by the pressure sensor 141 would drop. The reference symbol p2 denotes the inlet pressure of the second compression stage, which is measured by the pressure sensor 142.

The reference symbol κ denotes the ratio of specific heats of the adiabatic change and the reference symbol V_(stat) denotes a static clearance volume of the first compression stage, which results from the dimensions of the piston and the cylinder. The reference symbol d ultimately denotes the diameter of the piston of the first compression stage. In other words, the clearance volume of the first compression stage is increased due to the stroke reduction.

If the pressure on the inlet side of the second compression stage increases, the value Δh decreases such that the effective stroke is extended. Consequently, a large clearance volume has to be accepted for lower pressures in order to prevent compromising the inlet pressure monitoring during the reexpansion.

In order to minimize the computing effort, it is very practical to record corresponding values in tables, e.g. in 0.5 bar increments, and to store these tables in the SPC. In practical applications, such an incremental shutdown is then only activated starting at the inlet pressure, at which the electric linear motor is no longer able to set the piston in motion. 

1. A method for compressing a gas by means of a multi-stage piston compressor, wherein the gas is diverted at least partially upstream of the first compression stage and fed to a second compression stage, which directly follows the first compression stage, if an inlet pressure of the first compression stage exceeds a threshold value.
 2. The method according to claim 1, wherein the stroke (h₁) of a piston of the piston compressor, which is assigned to the first compression stage, is furthermore reduced.
 3. The method according to claim 2, wherein the stroke (h₁) is reduced in dependence on a residual pressure after a reexpansion in the first compression stage and/or an inlet pressure of the second compression stage.
 4. The method according to claim 3, wherein the reduction of the stroke (h₁) is determined based on stored values for the residual pressure and/or the inlet pressure of the second compression stage.
 5. The method according to claim 1, wherein the gas is diverted upstream of the first compression stage and fed to the second compression stage in that a first valve in a first flow path leading to the first compression stage is at least partially closed and a second valve in a second flow path leading from the first flow path to the second compression stage is at least partially opened.
 6. The method according to claim 5, wherein an inlet valve of the second compression stage is used as the first valve.
 7. The method according to claim 1, wherein at least one electric linear motor is used for moving pistons in the piston compressor.
 8. A computing unit, for carrying out a method for compressing a gas by means of a multi-stage piston compressor, wherein the gas is diverted at least partially upstream of the first compression stage and fed to a second compression stage, which directly follows the first compression stage, if an inlet pressure of the first compression stage exceeds a threshold value.
 9. A multi-stage reciprocating piston compressor with a gas inlet, a first compression stage and a second compression stage, wherein a first valve is arranged in a first flow path leading to a gas inlet of the first compression stage, wherein a second flow path branches off the first flow path upstream of the first valve and leads to a gas inlet of the second compression stage, and wherein a second valve is arranged in the second flow path.
 10. The multi-stage reciprocating piston compressor according to claim 9, wherein the inlet valve of the first compression stage forms the first valve.
 11. The multi-stage reciprocating piston compressor according to claim 9 with at least one electric linear motor for moving pistons of the reciprocating piston compressor.
 12. The multi-stage reciprocating piston compressor according to claim 9 further comprising a computing unit for carrying out a method for compressing a gas by means of a multi-stage piston compressor, wherein the gas is diverted at least partially upstream of the first compression stage and fed to a second compression stage, which directly follows the first compression stage, if an inlet pressure of the first compression stage exceeds a threshold value.
 13. The method according to claim 1 wherein the gas is diverted completely upstream of the first compression stage. 